DESCRIPTION (Applicant's Abstract): Every day humans must make countless judgments about the causes of events. Among the many potential consequences, our causal judgments may affect the lives of others (e.g., we punish the defendant who caused the plaintiff's injury); our own physical health (e.g., we avoid substances that cause our allergic reaction); or our own mental health (e.g., we may blame ourselves for causing an unfortunate outcome). The proposed program of research investigates how humans evaluate causal efficacy. It attempts to find some common mechanisms underlying our ability to do such reasoning using a variety of tasks that are usually studied separately. Three interrelated series of experiments are proposed to answer the following questions: 1. Are causal efficacy judgment based on mutability? Mutability refers to whether one can imagine changing an earlier event so that the later outcome would not have occurred. The proposed experiments examine (a) whether mutability is necessary and sufficient for causality and (b) other possible relations between mutability and causality by having subjects make both kinds of judgments for several kinds of real life scenarios. 2. Are causal efficacy judgments based on how much an event changes the probability of the outcome? Although the answer is clearly "yes" when people have information about multiple occurrences of an outcome, it is not assumed that judgments are made this way when people have information about only one occurrence. The proposed experiments explore whether people do use that strategy for single occurrences; use of such a strategy may explain why people believe that controllable, exceptional, and morally bad actions are especially causal. 3. Are causal efficacy judgments made controlling for alternative potential causes? Often two potential causes of an effect covary and the proper evaluation of the causal efficacy of each must be made while holding the other cause constant. Experiments questioning whether people behave like scientists in this manner will dispense covariation information to subjects using either trial-by-trial presentations or summary statistical tables. Differences in performance across tasks and measures may reveal different learning systems at work. Understanding how humans make causal judgments has been of theoretical interest to philosophers, lawyers, and cognitive, social, developmental, and clinical psychologists. This research also has practical implications for education, the legal system, and cognitive therapy techniques.